Conventional constructions of motors in which ferrite magnets are positioned in stators and rotors are shown in FIGS. 8, 9 and 10.
That is to say, the construction in FIG. 8 is an example of a motor utilizing a cylindrical ferrite magnet 1 on the stator side. The magnet is fastened to the inner circumference of the cylindrical yoke 2, and a rotor (not shown here) is placed in space 3 in the inner circumference side of the magnet. Usually, for a motor with such construction using a strong magnetic radial anisotropic ferrite magnet is difficult for the reasons explained later, and an isotropic cylinder type ferrite magnet 1 is utilized. As such, the motor has a relatively low output.
The construction shown in FIG. 9 is an example of a motor using a pair of segment magnets, 1a and 1b on the stator side. Each magnet is fastened to the inner circumference of the yoke 2, and a rotor (not shown) is positioned in the opposing space 3 on the inner circumference side where the magnetic pole system. Other motors use multiple segment magnets according to the number of magnetic poles. In such segment magnets, since it is possible to use a strong magnetic radial anisotropic ferrite magnet, they are used in relatively high output motors.
The construction shown in FIG. 10 is an example of a motor wherein a pair of segment magnets 1c and 1d are used on the rotor side. Each magnet is fastened to the outer circumference of the magnet support 5 and positioned within a stator (not shown) of the specified shape to configure a motor. Segment magnets 1c and 1d are ferrite magnets as shown in FIG. 9, and since they are strong magnetic radial anisotropic ferrite magnets, the motor produced has a relatively high output.
However, recently even for high output motors, from the point of view of simplification assembly (making the assembly operation more efficient) and preventing cogging, it has been sought to find a motor construction which positions radial anisotropic cylinder type ferrite magnets which have equivalent or better magnetic properties than the above stated radial anisotropic ferrite magnets.
As a method of manufacturing such radial anisotropic cylinder type ferrite magnets, raw material powder such as Sr ferrite pulverized powder and Ba ferrite pulverized powder with an average particle size of less than 2 micro meter are molded into a cylindrical form using the dry method under a magnetic field and sintered. Due to the shrinkage factors for the circumferential and radial direction differs in sintering, internal stress accumulates to cause easy cracking, preventing the implementation of this method.
To prevent cracking in the sintering process, it has been proposed to use a mixture of 50.about.80 wt % Sr ferrite pulverized powder with an average particle size less than 2 micro meter and 50.about.20 wt % Ba ferrite isotropic granulated powder with a particle size of 14.about.200 mesh in the presence of a magnetic field end to mold it using the dry method (Patent Bulletin Heisei 1-48643).
However, the magnetic characteristic of radial anisotropic cylinder shape ferrite magnets industrially sintered utilizing this method have upper limits of Br=3.4 kG, .sub.B H.sub.C =2.9 kG, and (BH)max=2.6 MGOe, and these are not enough to satisfy recent high performance demands.
That is to say, without the availability of radial anisotropic cylinder shape ferrite magnets with strong magnetic properties, which are indispensable to realization of sought after high power out-put motors, it is difficult to satisfy this demand.
As explained shove, for example, one body high performance radial anisotropic cylinder shape ferrite magnets consisting of Sr ferrite were hard to manufacture, so that segment magnets which are generally less susceptible to cracking were manufactured from these compositions, and as shown in FIG. 9, a pair were positioned at opposing ends, or these magnets were assembled into cylinder shape.
Therefore, the utilization of multiple segment magnets not only complicated the assembly processes, but as shown in FIG. 9 when a pair were positioned at opposing ends, a large space is created between magnets in circumferential directions. Also, when magnets were assembled in a cylindrical shape, many patches (connecting parts) were unavoidably made in the circumferential directions; thus, creating the problem of coggings in motors.
Particularly when these motors were used as wiper motors and fan motors, etc., noise associated with coggings was generated; and it was necessary to consider the environmental impact for those who work close to such motors.
As the other manufacturing method for radial anisotropic cylinder shape ferrite magnets for motors, a special molding die has been proposed (Patent Bulleten Heisei 4-19684). That is to say, in order to avoid cracking when sintering, 2 or 3 pieces of shape edged protrusions are placed radially to the rod of extrusion molding die or the mandrel of the rolling device. When the cylindrical molding is slid toward the axis to be released, inside the inner circumference the cross sectional V shaped 2 or 3 cuts which are extended axially are formed; and when sintered, and particularly in the cooling process, the internal stress is all focused on these cuts to prevent micro crackings in other parts of the molding.
However, the above stated method prevents complex cracking, and to make repair by adhesives easy, for example, to obtain above stated segment magnets by making cuts and crack magnets at predetermined places by focussing the internal stress. Therefore, this is not the method to obtain a uniform cylindrical shape ferrite magnet, and it does not solve problems of segment magnets and their assembly.
This invention concerns the development of the high performance radial anisotropic cylinder shape ferrite magnets, and the objective is to be able to utilize the powder composition that has the strong magnetic properties; and to provide the high performance radial anisotropic ferrite magnets in one uniform body without having to assemble segment magnets and their manufacturing method. Also, this invention eliminates the problems mentioned above, it aims to provide motors that can make the assembly more efficient, and lower noise and the miniaturization possible.